Method of improving the oxidative stability of a lubricating composition

ABSTRACT

Method of improving the oxidative stability of a lubricating composition which is used to lubricate a spark ignition internal combustion engine, the spark-ignition engine being comprised within the powertrain of a hybrid electric vehicle, wherein the method comprises the step of introducing into the combustion chamber of the spark-ignition engine a gasoline composition wherein the gasoline composition comprises a hydrocarbon base fuel containing 10 to 20% v olefins, not greater than 5% v olefins of at least 10 carbon atoms, and not greater than 5% v aromatics of at least 10 carbon atoms, based on the base fuel, initial boiling point in the range 30 to 40° C., T10 in the range 45 to 57° C., T50 in the range 82 to 104° C., T90 in the range 140 to 150° C. and final boiling point not greater than 220° C.

FIELD OF THE INVENTION

This invention relates to a method of improving the oxidative stability of a lubricating composition which is used for lubricating a spark ignition combustion engine, the spark ignition combustion engine being housed in the powertrain of a hybrid electric vehicle.

BACKGROUND OF THE INVENTION

The rising costs of hydrocarbon-based fuels and increasing concern about the environmental effects of carbon dioxide emissions have resulted in a growing demand for motor vehicles that operate either partly or entirely on electrical energy.

Hybrid Electric Vehicles (HEV) make use of both electrical energy stored in re-chargeable batteries and the mechanical energy converted from fuel, usually hydrocarbon based, by a conventional internal combustion engine (ICE). The batteries are charged during driving operation by the ICE and also by recovering kinetic energy during deceleration and braking. This process is offered by a number of vehicle original equipment manufacturers (OEMs) for some of their vehicle models. HEVs typically provide a normal driving experience, with the principle advantage of improved fuel consumption in comparison to conventional ICE only vehicles. Plug-in Hybrid Electric Vehicles (PHEVs) have similar functionality to HEVs, but in this application the battery can also be connected to the mains electrical system for recharging when the vehicle is parked. PHEVs typically have larger battery packs than HEVs which affords some all-electric range capability. Dynamic driving will use electric power and ICE, though the area of operation using an internal combustion engine (ICE) for propulsion may be restricted to cruising and intercity driving. Consequently the fuel appetite of vehicles may well be different from that required currently for conventional ICE or HEV equipped vehicles. For vehicles based exclusively in an urban environment, the increased EV mode capacity and plug-in charging function further reduce the level of ICE activity. This can lead to significantly extended residence time for the fuel tank contents compared to HEV and conventional ICE vehicles.

Conventional ICE vehicles typically deliver about 600 km (400 miles) range for a propulsion system weight of about 200 kg and require a re-fill time of around 2 minutes. In comparison, it is considered that a battery pack based on current Li-ion technology that could offer comparable range and useful battery life would weigh about 1700 kg. The additional weight of the motor, power electronics and vehicle chassis would result in a much heavier vehicle than the conventional ICE equivalent.

In a conventional ICE vehicle, the engine torque and power delivery from the engine must cover the full range of vehicle operating dynamics. However, the thermodynamic efficiency of an internal combustion engine cannot be fully optimised across a wide range of operating conditions. The ICE has a relatively narrow dynamic range. Hence a major challenge for the vehicle manufacturers (OEMs) is to develop engine technologies and transmission systems that allow the engine torque and power delivery from the engine to operate over the full range of vehicle operating dynamics. Electrical machines on the other hand can be designed to have a very wide dynamic range, e.g., are able to deliver maximum torque at zero speed. This control flexibility is well recognised as a useful feature in industrial drive applications and offers potential in automotive applications. Within their operating envelope, electrical machines can be controlled using sophisticated electronics to give very smooth torque delivery, tailored to the demand requirements. However it may be possible to provide different torque delivery profiles that are more appealing to drivers. Hence this is likely to be an area of interest going forward for automotive designers. At higher speeds, electrical drive systems tend to be limited by the heat rejection capacity of the power electronics and the cooling system for the electric motor itself. Additional considerations for high torque motors at high speeds are associated with the mass of the rotating components, where very high centrifugal forces can be produced at high speeds. These can be destructive. In HEVs and PHEVs, the electric motor is therefore able to provide only some of the dynamic range. However, this can allow the efficiency of the ICE to be optimised over a narrower range of operation. This offers some advantages in terms of engine design.

Hence, current hydrocarbon fuels developed for a full range ICE may not be optimised or indeed beneficial for HEV or PHEV ICE units. Fuels have been formulated and regulated for conventional ICE vehicles for many years and may therefore be considered to have stabilised, with degrees of freedom in the formulation space well understood. The relatively recent introduction of hybrid technology presents an opportunity to consider the fuel formulation space from an entirely new perspective.

Further, in order to maximise efficient operation of a HEV or a PHEV ICE unit, consideration must also be given to the lubricating composition which is used to lubricate the ICE within the powertrain of a HEV or PHEV. Due to the different operating cycles in an HEV or PHEV ICE unit compared with a conventional ICE unit, the lubricating composition tends to be exposed to more extreme conditions and greater oxidative stresses in an HEV/PHEV environment.

It is already known that the automotive industry has designed specific requirements for lubricant compositions operating under certain driving conditions, such as the “Aunt Minnie” driving cycle (a driving cycle that simulates a vehicle being used infrequently for short-distance trips, without the engine fully warming up to optimal operating temperatures before shutoff in cold climates). In an HEV or PHEV ICE unit, there are frequent engine stops and starts such that the ICE is only used for short periods of time and doesn't fully warm up before shutting off. In general, the ICE in a hybrid vehicle will be more prone to the dangers of “Aunt Minnie” type drive cycles because the ICE will be used less for the same drive pattern. This means that the crank-case lubricant does not fully warm up in an HEV or PHEV which therefore presents severe conditions for oxidation of the lubricant. A decrease in oxidative stability of the lubricant can lead to increased engine deposits which in turn can lead to undesirable effects such as reduced fuel economy, and the like. As mentioned above, this problem of stop/start is more severe in HEVs and PHEVs than in conventional ICE units, and therefore careful consideration has to be given to improving the oxidative stability of the lubricant in a HEV/PHEV.

It would therefore be desirable to find ways of improving the oxidative stability of a lubricating composition in a PHEV/HEV vehicle in order to maximise the efficient operation of the PHEV/HEV vehicle.

At the same, the oxidative stability of the fuel composition also needs to be considered in the case of a HEV/PHEV.

WO2004/113476 discloses gasoline compositions meeting certain parameters whose use as a fuel in a spark ignition engine results in improved stability of engine crank case lubricant. However, there is no mention in this document of the use of such a fuel in an HEV or PHEV vehicle, or of the specific benefits of using such a fuel for hybrid vehicles.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of improving the oxidative stability of a lubricating composition which is used to lubricate a spark ignition internal combustion engine, the spark-ignition engine being comprised within the powertrain of a hybrid electric vehicle, wherein the method comprises the step of introducing into the combustion chamber of the spark-ignition engine a gasoline composition wherein the gasoline composition comprises a hydrocarbon base fuel containing 10 to 20% v olefins, not greater than 5% v olefins of at least 10 carbon atoms, and not greater than 5% v aromatics of at least 10 carbon atoms, based on the base fuel, initial boiling point in the range 30 to 40° C., T10 in the range 45 to 57° C., T50 in the range 82 to 104° C., T90 in the range 140 to 150° C. and final boiling point not greater than 220° C.

It has surprisingly been found that by selecting a gasoline composition meeting certain parameters the oxidative stability of the lubricating composition in a HEV or a PHEV is improved.

DETAILED DESCRIPTION OF THE INVENTION

Light olefin content together with the T₁₀ range of 38 to 60° C. are believed to be key parameters in achieving enhanced stability of engine lubricant (crank-case lubricant), in spark ignition internal combustion engines fuelled by gasoline compositions of the present invention which are comprised in the powertrain of a hybrid electric vehicle. Frequent engine stops and starts in a HEV and a PHEV where the ICE is only in use for some of the time and for short periods.means that the crank-case lubricant does not fully warm up and presents severe conditions for oxidation of the lubricant. The effects of these start/stop driving cycles are more severe in HEV/PHEV vehicles than they are in conventional ICE vehicles. High front-end volatility (low T₁₀) and specified olefin content are believed to result in reduction in blowby of harmful combustion gases into the engine crank-case.

By “not greater than 5% v olefins of at least 10 carbon atoms” and “not greater than 5% v aromatics of at least 10 carbon atoms” is meant that the hydrocarbon base fuel contains amounts of olefins having 10 carbon atoms or more and amounts of aromatics having 10 carbon atoms or more, respectively in the range 0 to 5% v, based on the base fuel.

Gasolines contain mixtures of hydrocarbons, the optimal boiling ranges and distillation curves thereof varying according to climate and season of the year. The hydrocarbons in a gasoline as defined above may conveniently be derived in known manner from straight-run gasoline, synthetically-produced aromatic hydrocarbon mixtures, thermally or catalytically cracked hydrocarbons, hydrocracked petroleum fractions or catalytically reformed hydrocarbons and mixtures of these. Oxygenates may be incorporated in gasolines, and these include alcohols (such as methanol, ethanol, isopropanol, tert.butanol and isobutanol) and ethers, preferably ethers containing 5 or more carbon atoms per molecule, e.g. methyl tert.butyl ether (MTBE) or ethyl tert.butyl ether (ETBE). The ethers containing 5 or more carbon atoms per molecule may be used in amounts up to 15% v/v, but if methanol is used, it can only be in an amount up to 3% v/v, and stabilisers will be required. Stabilisers may also be needed for ethanol, which may be used up to 5% to 10% v/v. Isopropanol may be used up to 10% v/v, tert-butanol up to 7% v/v and isobutanol up to 10% v/v.

It is preferred to avoid inclusion of tert.butanol or MTBE. Accordingly, preferred gasoline compositions of the present invention contain 0 to 10% by volume of at least one oxygenate selected from methanol, ethanol, isopropanol and isobutanol.

Theoretical modelling has suggested that inclusion of ethanol in gasoline compositions of the present invention will further enhance stability of engine lubricant, particularly under cooler engine operating conditions. Accordingly, it is preferred that gasoline compositions of the present invention contain up to 10% by volume of ethanol, preferably 2 to 10% v, more preferably 4 to 10% v, e.g. 5 to 10% v ethanol.

Gasoline compositions according to the present invention are advantageously lead-free (unleaded), and this may be required by law. Where permitted, lead-free anti-knock compounds and/or valve-seat recession protectant compounds (e.g. known potassium salts, sodium salts or phosphorus compounds) may be present.

The octane level, (R+M)/2, will generally be above 85.

Modern gasolines are inherently low-sulphur fuels, e.g. containing less than 200 ppmw sulphur, preferably not greater than 50 ppmw sulphur.

Hydrocarbon base fuels as define above may conveniently be prepared in known manner by blending suitable hydrocarbon, e.g. refinery, streams in order to meet the defined parameters, as will readily be understood by those skilled in the art. Olefin content may be boosted by inclusion of olefin-rich refinery streams and/or by addition of synthetic components such as diisobutylene, in any relative proportions.

Diisobutylene, also known as 2,4,4-trimethyl-1-pentene (Sigma-Aldrich Fine Chemicals), is typically a mixture of isomers (2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene) prepared by heating the sulphuric acid extract of isobutylene from a butene isomer separation process to about 90° C. As described in Kirk-Othmer, “Encyclopedia of Chemical Technology”, 4th Ed. Vol. 4, Page 725, yield is typically 90%, of a mixture of 80% dimers and 20% trimers.

Gasoline compositions as defined above may variously include one or more additives such as anti-oxidants, corrosion inhibitors, ashless detergents, dehazers, dyes, lubricity improvers and synthetic or mineral oil carrier fluids. Examples of suitable such additives are described generally in U.S. Pat. No. 5,855,629 and DE-A-19955651.

Additive components can be added separately to the gasoline or can be blended with one or more diluents, forming an additive concentrate, and together added to base fuel.

A preferred gasoline composition for use in the method of the present invention comprises one or more antioxidants in order to improve the oxidative stability of the gasoline composition. Any antioxidant additive which is suitable for use in a gasoline composition can be used herein. A preferred anti-oxidant for use herein is a hindered phenol, for example BHT (butylated hydroxy toluene). It is preferred that the gasoline composition comprises from 10 ppmw to 100 ppmw of antioxidant.

Preferred gasoline compositions used in the method of the present invention have one or more of the following features:—

(i) the hydrocarbon base fuel contains at least 10% v olefins, (ii) the hydrocarbon base fuel contains at least 12% v olefins, (iii) the hydrocarbon base fuel contains at least 13% v olefins, (iv) the hydrocarbon base fuel contains up to 20% v olefins, (v) the hydrocarbon base fuel contains up to 18% v olefins, (vi) the base fuel has initial boiling point (IBP) of at least 28° C., (vii) the base fuel has IBP of at least 30° C., (viii) the base fuel has IBP up to 42° C., (ix) the base fuel has IBP up to 40° C., (x) the base fuel has T₁₀ of at least 42° C., (xi) the base fuel has T₁₀ of at least 45° C., (xii) the base fuel has T₁₀ of at least 46° C., (xiii) the base fuel has T₁₀ up to 58° C., (xiv) the base fuel has T₁₀ up to 57° C., (xv) the base fuel has T₁₀ up to 56° C., (xvi) the base fuel has T₁₀ of at least 80° C., (xvii) the base fuel has T₁₀ of at least 82° C., (xviii) the base fuel has T₁₀ of at least 83° C., (xix) the base fuel has T₁₀ up to 105° C., (xx) the base fuel has T₁₀ up to 104° C., (xxi) the base fuel has T₁₀ up to 103° C., (xxii) the base fuel has T₉₀ at least 135° C., (xxiii) the base fuel has T₉₀ of at least 140° C., (xxiv) the base fuel has T₉₀ of at least 142° C., (xxv) the base fuel has T₉₀ up to 170° C., (xxxi) the base fuel has T₉₀ up to 150° C., (xxvii) the base fuel has T₉₀ up to 145° C., (xxviii) the base fuel has T₉₀ up to 143° C., (xxix) the base fuel has final boiling point (FBP) not greater than 200° C., (xxx) the base fuel has FBP not greater than 195° C., (xxxi) the base fuel has FBP not greater than 190° C., (xxxii) the base fuel has FBP not greater than 185° C., (xxxvii) the base fuel has FBP not greater than 180° C., (xxxiv) the base fuel has FBP not greater than 175° C., (xxxv) the base fuel has FBP not greater than 172° C., (xxxvi) the base fuel has FBP of at least 165° C., and (xxxvii) the base fuel has FBP of at least 168° C.

Examples of preferred combinations of the above features include (i) and (iv); (ii) and (v); (iii) and (v); (vi), (viii), (x), (xii), (xvi), (xix), (xxii), (xxv) and (xxix); (vii), (ix), (xi), (xiv), (xvii), (xx), (xxiii), (xxvi) and (xxxiii); and (vii), (ix), (xii), (xv), (xviii), (xxi), (xxiv), (xxviii), (xxxvi) and (xxxvii).

Use of the gasoline composition described herein as fuel for a spark-ignition engine in a PHEV or HEV can give one of a number of benefits in addition to providing improved stability of engine lubricant (crank-case lubricant). These benefits include reduced frequency of oil changes, reduced engine wear, e.g. engine bearing wear, engine component wear (e.g. camshaft and piston crank wear), improved acceleration performance, higher maximum power output, and/or improved fuel economy.

Accordingly, the invention additionally provides the use of a gasoline composition as defined above as a fuel for a spark-ignition engine for improving oxidative stability of engine crank case lubricant and/or for reducing frequency of engine lubricant changes, wherein the spark-ignition engine is comprised in the powertrain of a hybrid electric vehicle.

The invention will be understood from the following illustrative examples, in which, unless indicated otherwise, temperatures are in degrees Celsius and parts, percentages and ratios are by volume. Those skilled in the art will readily appreciate that the various fuels were prepared in known manner from known refinery streams and are thus readily reproducible from a knowledge of the composition parameters given.

In the examples, oxidative stability tests on lubricant in engines fuelled by test fuels were effected using the following procedure.

A bench engine, Renault Megane (K7M702) 1.6 1, 4-cylinder spark-ignition (gasoline) engine was modified by honing to increase cylinder bore diameter and grinding ends of piston rings to increase butt gaps, in order to increase rate of blow-by of combustion gases. In addition, a by-pass pipe was fitted between cylinder head wall, above the engine valve deck, and the crankcase to provide an additional route for blow-by of combustion gases to the crank case. A jacketed rocker arm cover (RAC) was fitted to facilitate control of the environment surrounding the engine valve train.

Before test and between each test, the engine was cleaned thoroughly, to remove all trace of possible contamination. The engine was then filled with 15 W/40 engine oil meeting API SG specification, and the cooling systems, both for engine coolant and RAC coolant, were filled with 50:50 water:antifreeze mixture.

Engine tests were run for 7 days according to a test cycle wherein each 24 hour period involved five 4-hour cycles according to Table 1:—

TABLE 1 Control Parameters Stage 1 Stage 2 Stage 3 Duration (mins) 120 75 45  Speed (rpm) 2500 ± 11  2500 ± 11   850 ± 100 Torque (Nm) 70 ± 3 70 ± 3 0 Oil inlet ° C. 69 ± 2 95 ± 2 46 ± 2 Coolant ° C. 52 ± 2 85 ± 2 46 ± 2 RAC inlet ° C. 29 ± 2 85 ± 2 29 ± 2 followed by an oil sampling cycle wherein Stage 3 of Table 1 was replaced by a modified stage in which during a 10 min idle period (850±100 rpm) a 25 g oil sample was removed. (Every second day and on the seventh day (only) was sample removed). The engine was then stopped and allowed to stand for 20 minutes. During the next 12 minutes the oil dipstick reading was checked and engine oil was topped up (only during test, not at end of test). During the final 3 minutes of this 45-minute stage the engine was restarted.

Test measurements on oil samples were made to assess heptane insolubles (according to DIN 51365 except that oleic acid was not used as coagulant), total acid number (TAN)(according to IP177), total base number (TBN)(according to ASTM D4739), and amounts of wear metals (Sn, Fe and Cr) (according to ASTM 5185 except that sample was diluted by a factor of 20 in white spirit, instead of a factor of 10). From the TAN and TBN values (units are mg KOH/g lubricant), TAN/TBN crossover points were calculated (test hours).

Example 1

Three hydrocarbon base fuel gasolines were tested. Comparative Example A was a base fuel as widely employed in fuels sold in The Netherlands in 2002. Comparative Example B corresponded to Comparative Example A with addition of heavy platformate (the higher boiling fraction of a refinery steam manufactured by reforming naphtha over a platinum catalyst), to increase aromatics. Example 1 corresponded to Comparative Example A, with addition of light cat-cracked gasoline (the lower boiling fraction of a refinery stream produced by catalytic cracking of heavier hydrocarbons), to increase olefins. Sulphur contents of the fuels were adjusted to 50 ppmw S by addition, where necessary, of dimethylsulphide, in order to eliminate possible effects arising from differences in sulphur levels.

The resulting fuels had properties as given in Table 2:—

TABLE 2 Comparative Comparative Base Fuel Example 1 Example A Example B Density at 15° C. 0.7216 0.7316 0.754 DIN 51757/V4 RVP (mbar) 561 512 672 Distillation (ISO 3405/88) IBP (° C.) 30 32.5 35 10% 46 49.5 54 50% 83.5 107.5 109.5 90% 143 147.5 168.5 FBP 168.5 173 205.5 S(ASTM D 2622-94) 50 50 50 (ppmw) Paraffins (% v) 52.86 64.19 53.79 Olefins (% v) 16.4 0.61 0.43 Olefins of C10 0.00 0.00 0.00 or greater % v) Naphthenes (% v) 2.87 2.88 4.1 (saturated) Aromatics (% v) 27.01 31.41 40.74 Aromatics of C10 0.46 0.57 7.10 or greater (% v) Oxygenates 0 0 0 RON 95.3 96.1 95.8 MON 85.3 87.7 86.6

Results of tests on these fuels are given in Table 3:—

TABLE 3 Comparative Comparative Base Fuel Example 1 Example A Example B TAN/TBN cross- 101 47 50 over (hours) Wear Metals (mg metal/g lubricant) Cr (after 96 hours) less than 1 less than 1 less than 1 Cr (after 7 days) less than 1 less than 1 less than 1 Fe (after 96 hours) 14 15 17 Fe (after 7 days) 18 23 22 Sn (after 96 hours) 4  8 14 Sn (after 7 days) 4 11 15

The point at which TAN/TBN crossover occurs is considered to be an indicator of the point at which significant oxidative change is occurring in the oil.

The above results give a good indication that use of the fuel of Example 1 had a highly beneficial effect on oxidative stability of the crank case lubricant, leading to extended lubricant life, lower frequency of engine lubricant changes (extended service intervals), and reduced engine wear.

Tin levels are most likely to be associated with wear in engine bearings. Iron levels are associated with engine component wear (camshaft and piston cranks).

Examples 2 and 3

Four hydrocarbon base fuel gasolines were tested. Comparative Example C was a base fuel as widely employed in fuels sold in The Netherlands in 2002. Comparative Example D corresponded to Comparative Example C with addition of heavy platformate, to increase aromatics. Example 1 corresponded to Comparative Example C, with addition of 15 parts by volume diisobutylene per 85 parts by volume base fuel of Comparative Example C. The diisobutylene was a mixture of 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, in proportions resulting from commercial manufacture. Example 3 corresponded to Comparative Example C, with addition of an ex-refinery stream of C₅ and C₆-olefins, in proportion of 15 parts by volume olefins per 85 parts by volume base fuel of Comparative Example C.

The resulting fuels had properties as given in Table 4:—

TABLE 4 Comparative Comparative Base Fuel Example 2 Example 3 Example C Example D Density at 15° C. 0.7263 0.7232 0.7321 0.7557 DIN 51757/V4 RVP (mbar) 516 625 561 508 Distillation (ISO 3405/88) IBP (° C.) 35 32 35 35 10% 56 46.5 51.5 57 50% 102.5 87.5 105.5 105.5 90% 142 143 146 166 FBP 172 170.5 174.5 196.5 S (ASTM D 23 23 24 14 2622-94) (ppmw) paraffins (% v) 57.08 55.6 64.25 53.63 olefins (% v) 17.97 17.63 3.33 1.92 olefins of C10 0.00 0.00 0.00 0.00 or greater (% v) naphthenes (% v) 2.74 1.93 1.89 4.14 (saturated) aromatics (% v) 22.21 24.84 28.2 40.3 aromatics of C10 0.57 0.98 1.33 6.83 or greater (% v) oxygenates 0 0 0 0 RON 98.5 96.2 96.1 95.9 MON 87.6 85.9 87.7 86.5

Results of tests on these fuels are given in Table 5:—

TABLE 5 Compar- Compar- ative ative Base Fuel Example 2 Example 3 Example C Example D TAN/TBN cross- 100 127 100 68 over (hours) Wear Metals (mg metal/g lubricant) Cr (after 96 hours) less than 1 less than 1 less than 1 3 Cr (after 7 days) less than 1 less than 1 less than 1 4 Fe (after 96 hours) 9 12 12 16 Fe (after 7 days) 11 13 16 21 Sn (after 96 hours) 5 5 8 4 SN (after 7 days) 6 6 10 6 Heptane insolubles 0.08 0.08 0.11 0.42 (after 96 hours) (% w/w) Heptane insoluble 0.14 0.23 0.24 0.83 (after 7 days) (% w/w)

The above results overall give a good indication that use of the fuels of Examples 2 and 3 give overall unexpected benefits on oxidative stability of the crank case lubricant, with similar consequences as described above in Example 1.

Example 4

A fuel similar to Comparative Example C (Comparative Example E) was blended with diisobutylene and ethanol to give a gasoline composition containing 10% v/v diisobutylene and 5% v/v ethanol (Example 4). The resulting gasoline contained 13.02% v olefins, had initial boiling point 40° C., final boiling point 168.5° C., and met the other parameters of the present invention. This fuel was tested in a Toyota Avensis 2.0 1 VVT-i direct injection spark-ignition engine relative to Comparative Example E and relative to the same base fuel containing 5% v/v ethanol (Comparative Example F). Both Comparative Example E and Comparative Example F are outside the parameters of the present invention by virtue of their olefin contents (total olefins of 3.51% v/v and 3.33% v/v, respectively). Details of the fuels are given in Table 6:—

TABLE 6 Compar- Compar- ative ative Base Fuel Example 4 Example E Example F Density at 15° C. 0.7348 0.7333 0.7359 DIN 51757/V4 Distillation (ISO 3405/88) IBP (° C.) 40 38 35.5 10% 52.5 55 50 50% 100.5 101 97.5 90% 138.5 142 141 FBP 168.5 169 167 S (IP 336/95) 26 26 25 (ppmw) paraffins (% v) 52.16 61.36 58.1 olefins (% v) 13.02 3.51 3.33 olefins of C10 0 0 0 or greater (% v) naphthenes (% v) 2.13 2.58 2.49 (saturated) aromatics (% v) 26.62 31.93 30.15 aromatics of C10 0.49 0.59 0.55 or greater (% v) oxygenates 5.54 0 5.47 RON 99.7 95.2 97.5 MON 87.8 87.1 87.6

Under acceleration testing (1200-3500 rpm, 5th gear, wide open throttle (WOT), 1200-3500 rpm, 4th gear, WOT, and 1200-3500 rpm, 4th gear 75% throttle), Example 4 gave consistently superior performance (acceleration time) relative to either of Comparative Examples E and F. Significantly higher power was developed both at 1500 rpm and at 2500 rpm when the engine was fuelled with Example 4, relative to Comparative Example E or Comparative Example F. 

1. A method of improving the oxidative stability of a lubricating composition which is used to lubricate a spark ignition internal combustion engine, the spark-ignition engine being comprised within the powertrain of a hybrid electric vehicle, wherein the method comprises the step of introducing into the combustion chamber of the spark-ignition engine a gasoline composition wherein the gasoline composition comprises a hydrocarbon base fuel containing 10 to 20% v olefins, not greater than 5% v olefins of at least 10 carbon atoms, and not greater than 5% v aromatics of at least 10 carbon atoms, based on the base fuel, initial boiling point in the range 30 to 40° C., T10 in the range 45 to 57° C., T50 in the range 82 to 104° C., T90 in the range 140 to 150° C. and final boiling point not greater than 220° C.
 2. The method according to claim 1 wherein the gasoline composition contains 0 to 10% v of at least one oxygenate selected from methanol, ethanol, isopropanol and isobutanol.
 3. The method according to claim 1 wherein the hydrocarbon base fuel contains 10 to 20% v olefins.
 4. The method according to claim 1 wherein the hydrocarbon base fuel contains 12 to 18% v olefins.
 5. The method according to claim 1 wherein the base fuel has initial boiling point in the range 28 to 42° C., T₁₀ in the range 42 to 58° C., T₅₀ in the range 80 to 105° C., T₉₀ in the range 135 to 170° C. and final boiling point not greater than 200° C.
 6. The method according to claim 1 wherein the base fuel has initial boiling point in the range 30 to 40° C., T10 in the range 45 to 57° C., T50 in the range 82 to 104° C., T90 in the range 140 to 150° C., and final boiling point not greater than 180° C.
 7. The method according to claim 1 wherein the gasoline composition contains up to 10% v of ethanol.
 8. The method according to claim 1 wherein the fuel composition comprises one or more antioxidants.
 9. The method according to claim 1 wherein the lubricating composition comprises an antioxidant of the hindered phenol type.
 10. The method according to claim 1 wherein the hybrid electric vehicle is a plug-in hybrid electric vehicle.
 11. A gasoline composition suitable for use as a fuel for a spark-ignition engine for improving oxidative stability of engine crank case lubricant and/or for reducing frequency of engine lubricant changes, wherein the spark-ignition engine is comprised in the powertrain of a hybrid electric vehicle, wherein the gasoline composition comprises a hydrocarbon base fuel containing 10 to 20% v olefins, not greater than 5% v olefins of at least 10 carbon atoms, and not greater than 5% v aromatics of at least 10 carbon atoms, based on the base fuel, initial boiling point in the range 30 to 40° C., T10 in the range 45 to 57° C., T50 in the range 82 to 104° C., T90 in the range 140 to 150° C. and final boiling point not greater than 220° C. 